In recent years, the world demand for fuels of all types has greatly expanded. In order to find additional supplies of such fuels as oil and natural gas, it has been increasingly necessary to turn to subsea exploration. As exploration for oil and gas moved into coastal waters, the initial attempts to drill oil and gas wells at subsea locations utilized adaptations of conventional land based drilling techniques. Thus, for example, before drilling began, a stable drilling platform was generally established by mounting the platform on legs that extended down to the ocean floor. Nonetheless, as the search for deposits of petroleum and natural gas has extended into deeper waters farther from shore, it has become necessary to abandon fixed drilling platforms and to turn to floating platforms or barges from which to conduct drilling. In some instances, floating platforms have also been used to mount equipment for pumping oil or natural gas from a producing well.
A floating drilling platform, barge, or other structure is particularly susceptible to movements in response to wave action, even though the platform or barge may be anchored. Drilling or pumping operations that are conducted from a floating platform must accommodate both lateral and vertical movements of the platform. Accordingly, drilling strings, riser lines, and similar conduits which extend downwardly from a drilling platform to the ocean floor must be capable of bending sufficiently to avoid rupturing when the drilling platform moves slightly from its designated location. Typically, the pipe that is used in a drilling string, for example, is of a sufficiently small diameter and has sufficient strength to be flexible enough to avoid damage when an associated drilling platform moves laterally or vertically. A riser line or marine conductor pipe, on the other hand, has a relatively large diameter and, therefore, a greater rigidity than a drill string. As a result, large diameter riser pipes or pumping lines must typically include at least one coupling or joint assembly that can be readily flexed to accommodate bending motion yet can provide a seal adequate to withstand high internal and external fluid pressures.
One type of flexible joint used in riser pipes consists of a ball member having a precisely machined spherical surface and a socket member having a complementary spherical surface. The joint is flexed by sliding one of the spherical surfaces relative to the other. Resilient O-rings help seal the joint at the interface between the sliding surfaces. The flexural movement of such a ball joint is impaired, however, when the joint is subjected to high pressures. The joint is also subject to frictional wear and deterioration of both the sliding surfaces and the O-ring seals. The frictional wear requires frequent repair or replacement of the joint.
Another type of flexible joint for fluid conduits, such as marine riser pipes, utilizes annular flexible elements disposed between flanges secured to adjacent ends of different sections of conduit. The flexible elements generally comprise alternating layers of a nonextensible material and a resilient material, which are normally metal and an elastomer. The layers or laminations may be annular with flat surfaces, as in the pipe joint of Johnson U.S. Pat. No. 3,168,334, or annular with spherical surfaces, as in the flexible joint of Herbert et al U.S. Pat. No. 3,680,895. Laminated flexible elements permit the necessary flexural movements of a joint and can also function as seals. A joint incorporating a laminated element has no "moving" parts and is not subject to the frictional wear encountered with the ball-and-socket joints discussed above. Other flexible pipe joints utilizing laminated flexible elements are described and illustrated in Herbert et al U.S. Pat. Nos. 3,390,899, 3,734,546, and 3,853,337.
A laminated flexible element, such as the elements described above, accommodates motion, including relative pivotal or cocking motion between adjacent lengths of conduit, through torsional shearing of the elastomer or other resilient material within the element. Any flexible element that relies on shearing of elastomer to accommodate motion is limited in its motion accommodation by the height or thickness of the elastomer that is being sheared. At any given time, a body of elastomer can be strained in shear 300% or more without adverse effects. Thus, one end of a body of elastomer that is 2 inches thick can be moved 6 inches or more relative and parallel to the other end of the elastomeric body without rupture or other failure of the elastomer. Nonetheless, if the body of elastomer were cycled through a multiplicity of shear loadings, the continued application and release of a strain on the order of 300% would shortly result in fatigue failure of the elastomer. In situations, such as a coupling for a marine riser, in which the elastomer of a flexible element will be subjected to millions of cycles of shear loading, it is desirable to limit the maximum strain on the elastomer to low levels, a typical range of maximum strains being 25-35%. The precise value or range of values for the maximum strain in a given situation will depend upon the duty cycle for the flexible element (i.e., the various sizes of motion to be accommodated and the frequency of each size) and upon the fatigue characteristics for the particular elastomeric material used in the flexible element. Consequently, in order to avoid detrimental increases in maximum strain and resultant decreases in fatigue life, the height or thickness of a body of elastomer must be increased if it is to accommodate increased motion.
Although the accommodation of torsional motion through shearing of the elastomer in a laminated flexible element does not follow precisely the same principles as the accommodation of translational motion through elastomer shearing, it is generally true that increases in the height or thickness of a body of elastomer will permit the elastomer to accept a greater degree of torsional motion. The ability to permit pivotal or torsional movements on the order of 10.degree. to 15.degree. in any direction from a neutral axis is an important characteristic of a flexible joint assembly for a marine riser pipe, for example. Thus, it is not surprising to see a flexible pipe joint assembly that incorporates a relatively thick or tall laminated elastomeric element, such as is shown in FIG. 10 on page 6 of ASME paper 76-Pet-68. The pipe joint, which was discussed in an oral presentation given during the week of Sept. 19-24, 1976 in Mexico City, Mexico, is also being offered for sale by Oil States Rubber Company of Arlington, Tex. Despite its apparent value in a flexible pipe joint assembly, the use of a relatively tall or thick laminated elastomeric body to accommodate large pivotal motions has significant drawbacks. First, as is indicated in Schmidt U.S. Pat. No. 3,679,197, the effective spring rate of an element of rubber at any distance from a point about which pivotal motion occurs is proportional to the spring rate of the elastomer in translational shear multiplied by the square of the distance from the pivot point. Thus, for a laminated elastomeric element in which the elastomeric layers are concentrically arranged, the contribution of the radially outer elastomeric layers or laminations of the element to the total rotational stiffness of the element is considerably greater than the contribution of the radially inner layers or laminations. Since the inner elastomeric laminations are effectively much softer in rotational shear than the outer elastomeric laminations, the major portion of the torsional strain or deflection will occur in the radially inner laminations. Consequently, doubling the height or thickness of the elastomer utilized in a laminated element that is used to facilitate rotational movement will not double the motion accommodation of the flexible element for any given maximum strain. Instead, doubling the height may increase the rotational motion accommodation capability by as little as 25%. Increasing the height or thickness of a body of elastomer in order to accommodate additional rotational motion is feasible, therefore, but inefficient.
Another problem associated with increasing the height or thickness of a laminated elastomeric element is that the element becomes unstable and tends to tilt or buckle, as is described in Peterson U.S. Pat. No. 3,292,711, particularly at column 2, lines 6 to 46. Still a third problem associated with using a relatively thick or tall laminated elastomeric element is that flexing of the element to provide rotational motion accommodation may leave substantial portions of each lamination unsupported. As a result, a compression load, for example, may be applied to a portion of a laminated element without that portion of the element being adequately supported by an appropriately rigid supporting structure. The unsupported loading of the laminations in the element, particularly the non-extensible laminations, will cause bending and, ultimately, failure of the non-extensible laminations. Failure of the nonextensible laminations effectively means failure of the flexible element as a whole.
The buckling problem that is mentioned above can be overcome to some extent by interposing within the length or height of the laminated flexible element, at one or more locations, an extra thick and rigid lamination or shim. This technique has been suggested in French Pat. No. 934,336, particularly at FIG. 6, and in Irwin U.S. Pat. No. 3,504,902, particularly at column 3, lines 56-68. The use of such an extra thick and rigid shim to overcome the buckling problem might also help to overcome the problems associated with unsupported portions of the nonextensible laminations in the flexible element.